Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
PSYCHROMETRIC CHART FUNDAMENTALS
44
1. Draw diagonal lines parallel to the constant enthalpy lines
from Points A and B to the enthalpy scale.
2. Read the enthalpy on the enthalpy scale.
3. Calculate the enthalpy added as follows:
Total heat at Point B – total heat at Point A =
total heat added.
40.0 – 22.8 = 17.2 kilojoules per kilogram of
dry air
Since there is no change in moisture content, the total heat added
is all sensible. Whenever the process moves along a constant
moisture line, only sensible heat is changed.
COOLING PROCESS
The cooling process removes sensible heat and, often, latent
heat from the air. Consider a condition where only sensible
heat is removed. Figure 12 illustrates a cooling process where
air is cooled from 32
°
C to 21
°
C but no moisture is removed.
Line A-B represents the process line for cooling. The relative
humidity in this example increases from 50 percent (Point A)
to 95 percent (Point B) because air at 21
°
C cannot hold as much
moisture as air at 32
°
C. Consequently, the same amount of
moisture results in a higher percentage relative humidity at 21
°
C
than at 32
°
C. Calculate the total heat removed as follows:
Fig. 12.
Total heat at Point A - total heat at Point B =
total heat removed.
70.5 – 59.0 = 11.5 kilojoules per kilogram of
dry air
This is all sensible heat since there is no change in moisture
content.
HUMIDIFYING PROCESS
BASIC PROCESS
The humidifying process adds moisture to the air and crosses
constant moisture lines. If the dry bulb remains constant, the
process involves the addition of latent heat only.
Relative humidity is the ratio of the amount of moisture in
the air to the maximum amount of moisture the air can hold at
the same temperature and pressure. If the dry-bulb temperature
increases without adding moisture, the relative humidity
decreases. The psychrometric charts in Figures 13 and 14
illustrate what happens. Referring to Chart No. 2 (Fig. 13),
outdoor air at –18
°
C dry bulb and 75 percent rh (Point A)
contains about 0.55 grams of moisture per kilogram of dry air.
The 0.55 grams of moisture per kilogram of dry air is carried
over to Chart No. 1 (Fig. 14) and a horizontal line (constant
moisture line) is drawn.
Fig. 13. Chart No. 2.
B
A
C4328
32
°
C DB
50% RH
SUPPLY FAN
COOLING COIL
AIRFLOW
21
°
C DB
95% RH
70.5 kJ/kg
59 kJ/kg
21
°
C DB
95% RH
32
°
C DB
50% RH
C4329
A
–18
°
C DB
75% RH
0.55 g/kg